1) Field of the Invention
The present invention relates to vehicle mirrors, and more particularly to exterior convex rearview vehicle mirrors.
2) Description of Related Art
Curved rearview mirrors, often referred to as spot-mirrors, that are typically used around a vehicle exterior have a spherical convex shape with a constant curvature. The convex mirror provides a wider field of view than a standard flat mirror surface.
There are also rearview mirrors that feature a surface with varying curvatures where the curvature typically increase towards the outboard edge of the mirror. these types of varying curvature mirrors provide a wider field of view than a plain constant curvature convex mirror, but also exhibit extreme distortion towards the outboard edge as the curvature increases. However, referring to FIG. 5, even a simple convex mirror with a spherical constant radius of curvature distorts the perceived image significantly. This distortion gets stronger when viewing the mirror surface at an angle. For rearview vehicle mirrors, this means that the worst case for viewing a convex mirror is on the right hand side of the vehicle since the driver looks onto the mirror surface more from the side instead of perpendicular to the mirror surface.
With reference to FIG. 6, to counteract the natural distortion of spherical convex mirrors while maintaining the increased field of view, several attempts have been described in the prior art that relate to dual radius mirror designs. The dual radius design involves curving the mirror surface along two different axis of curvatures with different radius of curvatures. A first axis is defined as extending heightwise relative to the mirror surface, and a second axis is defined as extending widthwise relative to the mirror surface. The radius of curvature extending heightwise along the first axis is typically smaller than the radius of curvature extending widthwise along the second axis. This can be envisioned by having two circle portions, one smaller than the other with the smaller circle defining a plane aligned vertically along the first axis, and a second circle defining a plane aligned horizontally. The circles are arranged so they intersect each other with the associated planes of the circles perpendicular to each other and the center of the mirror face being located at the intersecting perimeter of the circles. The mirror surface is created by sweeping the smaller radius circle along the larger radius circle in a way that the associated plane of the smaller circle is always perpendicular to the larger circle's curve. Thus, the first axis of curvature is arranged perpendicular to the second axis of curvature.
The image produced by utilizing a dual radius mirror surface of the type known in the prior art more faithfully maintains the width and height proportions of the targeted objects as opposed to a spherical convex mirror or convex mirror with varying radius of curvature. However, the shape still exhibit a “sheer-distortion” making objects appear to be heavily slanted and leaning outboard.
Accordingly, it is an object of the present invention to reduce distortion associated with a convex mirror while providing a vehicle driver with a wider field of view than a flat mirror surface.
It is another object of the present invention to balance the field of view available to a driver between left and right exterior rearview mirror assemblies.